Nowadays, spontaneous ignition of metal powders causes serious accidents in the metal powder industry. It is known that the ignition is caused by heat accumulation. Therefore, it is important to find the cause of heat accumulation of metal powder to prevent such accidents. In this study, the heat generation and heat accumulation of the chemical and/or physical reaction of metal powders with water were evaluated using the three calorimeters SuperCRC, ARC, and DSC. The correlation between the storage scale and the critical temperature of thermal runaway of the reaction was also evaluated using the Frank-Kamenetskii model. According to the results of these experiments and the use of the Frank-Kamenetskii model, thermal runaway appears to be caused by the heat of the chemical and/or physical reaction of the metal powder with water. This suggests that the chemical and/or physical reaction of metal powders with water is one of the causes of spontaneous ignition.